Flash memory is a type of electronic memory media that can hold its data in the absence of operating power. Flash memory can be programmed, erased, and reprogrammed during its useful life (which may be up to one million write cycles for typical flash memory devices). Flash memory is becoming increasingly popular as a reliable, compact, and inexpensive nonvolatile memory in a number of consumer, commercial, and other applications. As electronic devices get smaller and smaller, it becomes desirable to increase the amount of data that can be stored per unit area on an integrated circuit memory cell, such as a flash memory unit.
One conventional flash memory technology is based upon a memory cell that utilizes a charge trapping dielectric cell that is capable of storing two bits of data. Non-volatile memory designers have recently designed memory circuits that utilize two charge storage regions to store charge within a single silicon nitride layer. This type of non-volatile memory device is known as a dual-bit Flash electrically erasable and programmable read-only memory (EEPROM), which is available under the trademark MIRRORBIT™ from Advanced Micro Devices, Inc., Sunnyvale, Calif. In such an arrangement, one bit can be stored using a first charge storing region on one side of the silicon nitride layer, while a second bit can be stored using a second charge storing region on the other side of the same silicon nitride layer. For example, a left bit and right bit can be stored in physically different areas of the silicon nitride layer, near left and right regions of each memory cell, respectively. In comparison to a conventional EEPROM cell, a dual-bit memory cell can store twice as much information in a memory array of equal size.
FIG. 1 is a cross-sectional view of a conventional dual-bit memory cell 50. The memory cell 50 has a dual-bit (bit1, bit2) architecture that allows twice as much storage capacity as a conventional EEPROM memory device.
The conventional memory cell 50 includes a substrate 54, a first insulator layer 62 disposed over the substrate 54, a nitride charge storage layer 64 disposed over the first insulator layer 62, a second insulator layer 66 disposed over the charge storage layer 64, and a polysilicon control gate 68 disposed over the second insulator layer 66. To produce an operable memory device, a first metal silicide contact (not shown) can be disposed on substrate 54, and the control gate 66 can be capped with a second metal silicide contact (not shown).
In one implementation, the substrate 54 is a P-type semiconductor substrate 54 having a first buried junction region 60 and a second buried junction region 61 formed within substrate 54 in self-alignment with the memory cell 50. First buried junction region 60 and second buried junction region 61 are each formed from an N+ semiconductor material. The charge storage layer 64 is capable of holding a charge. The first insulator layer 62, the charge storage layer 64, and the second insulator layer 66 can be implemented using an oxide-nitride-oxide (ONO) configuration in which a nitride charge storage layer 64 is sandwiched between two silicon dioxide insulator layers 62, 66. Alternatively, charge storage layer 64 may utilize buried polysilicon islands as a charge trapping layer.
Memory cell 50 can store two data bits: a left bit represented by the circle (bit 1); and a right bit represented by the circle (bit 2). In practice, memory cell 50 is generally symmetrical, thus first buried junction region 60 and second buried junction region 61 are interchangeable. In this regard, first buried junction region 60 may serve as the source region with respect to the right bit (bit 2), while second buried junction region 61 may serve as the drain region with respect to the right bit (bit 2). Conversely, second buried junction region 61 may serve as the source region with respect to the left bit (bit 1), while first buried junction region 60 may serve as the drain region with respect to the left bit (bit 1). A threshold voltage exists between the control gate 66 and the substrate 54 to prevent leakage during functioning of the device.
While a single dual-bit memory cell 50 is illustrated in FIG. 1, it will be appreciated that any suitable number of the dual-bit memory cells 50 could be used to form a memory array, as described below with reference to FIG. 2.
FIG. 2 is a simplified diagram of a plurality of dual-bit memory cells arranged in accordance with a conventional array architecture 200 (a practical array architecture can include thousands of dual-bit memory cells 50). Array architecture 200 includes a number of buried bit lines formed in a semiconductor substrate as mentioned above. FIG. 2 depicts three buried bit lines (reference numbers 202, 204, and 206), each being capable of functioning as a drain or a source for memory cells in array architecture 200. Array architecture 200 also includes a number of word lines that are utilized to control the gate voltage of the memory cells. FIG. 2 depicts four word lines (reference numbers 208, 210, 212, and 214) that generally form a crisscross pattern with the bit lines. Although not shown in FIG. 2, charge storage layer, such as an ONO stack, resides between the bit lines and the word lines. The dashed lines in FIG. 2 represent two of the dual-bit memory cells in array architecture 200: a first cell 216 and a second cell 218. Notably, bit line 204 is shared by first cell 216 and second cell 218. Array architecture 200 is known as a virtual ground architecture because ground potential can be applied to any selected bit line and there need not be any bit lines with a fixed ground potential.
Control logic and circuitry (not shown) for array architecture 200 governs the selection of memory cells, the application of voltage to the word lines 208, 210, 212, 214, and the application of voltage to the bit lines 202, 204, 206 during conventional flash memory operations, such as: programming; reading; erasing; and soft programming. Voltage is delivered to the bit lines 202, 204, 206 using bit line contacts (not shown). FIG. 2 depicts three conductive metal lines (reference numbers 220, 222, and 224) and three bit line contacts (reference numbers 226, 228, and 230). For a given bit line, a bit line contact is used once every 16 word lines because the resistance of the bit lines is very high.
When charging up the charge storage layer 64, one way to reduce or minimize power consumption is by using Fowler-Nordheim (FN) tunneling mechanism to inject electrons into the charge storage layer 64 and thereby erase the memory cell 50.
FIG. 3 is a cross-sectional view of the conventional dual-bit memory cell during a Fowler-Nordheim (FN) erase operation in which FN tunneling can be used to inject electrons into the nitride charge storage layer 64. The basic structure of the memory cell 50 is described above with respect to FIG. 1, and for sake of brevity will not be described again. The buried junction regions 60, 61 can either be floating or grounded. The highly positive gate 68 bias voltage (e.g., 18 volts to 20 volts) pulls electrons (Θ) from the grounded substrate 54 into the charge storage layer 64 such that the charge storage layer 64 is evenly charged with electrons (Θ). This FN tunneling operation involves a relatively small amount of current and therefore consumes relatively low power.
FIG. 4 is a cross-sectional view of the structure of the conventional dual-bit memory cell during band-to-band hot hole (BTBHH) programming operation. The basic structure of the memory cell 50 is described above with respect to FIG. 1, and for sake of brevity will not be described again. This particular bias configuration, can be used to inject hot holes (electrically positively charged) into the right hand side (bit 2) of the nitride charge storage layer 64 to neutralize electrons stored at bit 2 thereby “programming” bit 2 of the memory cell 50. The right bit (bit 2) is programmed by applying a relatively high negative voltage to gate 68 via the appropriately selected word line, floating the bit line corresponding to first buried junction region 60 (which serves as the source in this case), and applying a medium positive bias voltage to the bit line corresponding to second buried junction region 61 (which serves as the drain in this case). This injects holes into the nitride layer 64 to neutralize electrons trapped in the nitride layer 64 at bit 2. Although not shown, by switching the drain/source biasing condition, holes can be injected into bit 1. The left bit (bit 1) is programmed by applying a relatively high negative voltage to gate 68 via the appropriately selected word line, floating the bit line corresponding to second buried junction region 61 (which serves as the source in this case), and applying a medium positive bias voltage to the bit line corresponding to first buried junction region 60 (which serves as the drain in this case).
FIG. 5 is a cross-sectional view of a conventional dual-bit memory cell 50 which illustrates residual electrons (Θ) that result in the center of the charge storage layer 64 as a result of the program operation. In an ideal situation, after programming one of the bits 1, 2 of the memory cell 50, the other one of the bits 2, 1 would contain exactly one-half of the electrons that were initially established in the charge storage layer 64 during the charge up operation of FIG. 3. In other words, in an ideal situation, when bit 1 is programmed, one-half of the electrons at bit 1 would be neutralized, when bit 2 is programmed, one-half of the electrons at bit 2 would be neutralized, and if both bit 1 and 2 are programmed the entire charge storage layer 64 would be neutralized. However, as illustrated in FIG. 5, band-to-band hot hole (BTBHH) programming leaves residual electrons (Θ) in the center portion of the charge storage layer 64 since hot holes can not be injected that far and therefore residual electrons (Θ) can not be neutralized. This results in degraded device operation or performance and reliability problems. For example, the residual electrons (Θ) in the center portion of the charge storage layer 64 can interfere with transistor operation since the transistor is no longer uniform because the threshold voltage at the center of the charge storage layer 64 would be different than the threshold voltages near the ends of the charge storage layer 64.
Notwithstanding these advances, it would be desirable to provide improved techniques for erasing and/or programming a dual-bit memory cell. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.